Hi Andy,
The runways in Istanbul are parallel. The latest ground chart I have shows they’re all on a magnetic heading of 354 degrees. The airport authority in association with the Turkish CAA has decided that this is the best way to number the runways. I think even more runways are planned which may see runway 18/36 become a L/R or C in the future too!
Paris CDG is another where the runways to the north of the central area are 09/27s and the southerly ones are 08/26s despite all being on the same headings.
Essentially, it’s a best practice to number them based on the magnetic heading but slight differences are allowed especially if runways are grouped to reduce the chance of confusion (I.e. 34L and R are next to each other, then 35L and R etc).